Techniques for processing image data generated from three-dimensional graphic models

ABSTRACT

Techniques are disclosed for creating animated video frames which include both computer generated elements and hand drawn elements. For example, a software tool may allows an artist to draw line work (or supply other 2D image data) to composite with an animation frame rendered from a three dimensional (3D) graphical model of an object. The software tool may be configured to determine how to animate such 2D image data provided for one frame in order to appear in subsequent (or prior) frames in a manner consistent with changes in rendering the underlying 3D geometry.

BACKGROUND

1. Field

Embodiments of the invention are generally directed to animated videosequences. More specifically, embodiments of the invention are directedto hybrid approaches for creating animated video from computerrenderings of three-dimensional (3D) models and in some cases hand-drawnline work supplied by animators.

2. Description of the Related Art

For many years, animated video has been created from a set ofindividually hand drawn frames. In recent years, computer softwareapplications have been developed that can also generate animated video.For example, software may provide an animator with an application usedto draw animation frames. Further, computer graphics applications may beused to create animated video from complex 3D models. For example, abroad variety of computer software and techniques have been developed torender computer generated images (CGI) from 3D models. Such approacheshave been very successful—both artistically and commercially—at creatinganimated video.

At the same time, hand drawn animation and CGI rendering each producedistinctive results. For example, in the case of hand-drawn animation,it is difficult for an animator to create consistent line work acrossmany frames, as small wobble or other variation in the individual linestends to appear from frame to frame. Further, the medium used for handdrawing (e.g., painting with a brush) tends to appear different fromframe to frame regardless of how consistent the artist is in paintingindividual lines. This effect is more apparent when an artist isanimating slow or subtle motion. In contrast, the shading, shadow anddepth created using CGI rendering produces can provide a very consistentappearance for subtle manipulations of curves. Of course, eitherapproach may be used to produce aesthetically pleasing results and anart director may desire to rely on one form of animation over another tocreate a particular effect.

SUMMARY

One embodiment disclosed herein includes a method for processing ananimation frame. This method may generally include identifying a firstanimation frame rendered by one or more processors. The first animationframe renders 3D geometry corresponding to a 3D model object at a firstpoint in time. This method may also include generating a screen spacevelocity map indicating a dynamic velocity of one or more coordinates inthe first animation frame relative to changes in points on the 3D modelobject occurring over one or more specified time steps.

In a particular embodiment, the one or more coordinates in the screenspace velocity map are mapped to pixel coordinates in the firstanimation frame. Further, the dynamic velocity of each coordinate mayinclude both a magnitude and direction of pixel movement. In anotherembodiment, this method may further include receiving 2D image data tocomposite with the first animation frame and animating the 2D image dataover the specified time step according to the dynamic velocity of theone or more coordinates in the screen space velocity map.

Still another embodiment includes a method for generating atwo-dimensional (2D) silhouette for geometry of a 3D graphical model.This method may generally include defining a continuous surfaceparameterization for at least a portion of the geometry representing the3D graphical model. A first dimension of the 2D surface parameterizationintersects a portion of the surface of the 3D graphical model. Thismethod may also include tracking a silhouette feature over one or moreframes. The tracking itself may include identifying a plurality ofpoints on the first dimension of the surface parameterization, and foreach of identified point, determining at least one second dimension ofthe surface parameterization. Each tangent point on the surface of the3D model is selected so as to be tangent to a viewpoint of a renderingcamera. Tracking the silhouette feature over the one or more frames mayinclude connecting the tangent points to produce geometry defining a 2Dsilhouette for the portion of the surface of the 3D graphical model ineach of the one or more frames.

In a particular embodiment, the first dimension of the 2D surfaceparameterization is determined as a cylindrical feature wrapping aportion of the surface of the 3D graphical model. Further, this methodmay also include deriving a plurality of closed loops generallyperpendicular to an axis of the cylindrical feature. In such a case,determining at least one second dimension of the surfaceparameterization may include identifying at least one point on theclosed loop tangent to the viewpoint of the rendering camera.

Other embodiments include, without limitation, a computer-readablemedium that includes instructions that enable a processing unit toimplement one or more aspects of the disclosed methods as well as asystem configured to implement one or more aspects of the disclosedmethods.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited aspects are attained andcan be understood in detail, a more particular description ofembodiments of the invention, briefly summarized above, may be had byreference to the appended drawings. It is to be noted, however, that theappended drawings illustrate only typical embodiments of this inventionand are therefore not to be considered limiting of its scope, for theinvention may admit to other equally effective embodiments. The patentor application file contains at least one drawing executed in color.Copies of this patent or patent application publication with colordrawing(s) will be provided by the Office upon request and payment ofthe necessary fee.

FIG. 1 illustrates a computing infrastructure configured to provide anartist with an animation tool, according to one embodiment of theinvention.

FIG. 2A illustrates an example of a character rendered from athree-dimensional (3D) model, according to one embodiment of theinvention.

FIGS. 2B-2C illustrate graphical representations of a pixel velocity mapcorresponding to the character rendered in the animation frame shown inFIG. 2A, according to one embodiment of the invention.

FIG. 3 illustrates a method for generating a pixel velocity map,according to one embodiment of the invention.

FIGS. 4A-4B illustrate example animation frames rendered from athree-dimensional (3D) model which include hand drawn 2D line workrendered using pixel velocity maps, according to one embodiment of theinvention

FIG. 5 illustrates a method for rendering hand drawn 2D line work usingpixel velocity maps, according to one embodiment of the invention.

FIG. 6 illustrates a method for generating a 2D silhouette for a 3Dobject rendered in a sequence of animation frames, according to oneembodiment of the invention.

FIGS. 7A-7B illustrate an example of generating a silhouette flange fora 3D model rendered in a sequence of animation frames, according to oneembodiment of the invention.

FIG. 8 illustrates an example of tangent points being identified forpoints on a 3D model, according to one embodiment of the invention.

FIGS. 9A-9B illustrate an example of using points along a cylinder togenerate a silhouette flange for a 3D model rendered in two differentanimation frames, according to one embodiment of the invention.

FIGS. 10A-10B illustrate the silhouette flange shown for the 3D model inFIGS. 9A-9B from an alternative camera angle, according to oneembodiment of the invention.

FIGS. 11A-11B illustrate 2D line work generated from the silhouetteflange of the 3D model shown in FIGS. 9A-9B, according to one embodimentof the invention.

FIG. 12 illustrates an example computing system configured to implementembodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention provide techniques for creating animatedvideo frames which include both computer generated elements and handdrawn elements. For example, one embodiment includes a software toolwhich allows an artist to draw line work on an animation frame renderedfrom a three dimensional (3D) model. The software tool may be configuredto determine how to modify such hand drawn line work in one frame inorder to appear in subsequent (or prior) frames in a manner consistentwith changes in rendering the underlying 3D geometry.

In one embodiment, a screen space velocity map may be created to show adirection and velocity for point locations in a two dimensional (2D)screen space generated from a rendering of 3D geometry. Without loss ofgenerality, such points may correspond to pixels in an animation frameand the discussion below generally refers to pixels as a matter ofconvenience. More generally however, points in the 2D screen spacegenerated by rendering a 3D model may be sampled using any approachsuitable for the needs of a particular case. For example, a screen spacecould be rendered using a [0 . . . 1] interval, creating a screen spacevelocity map of an arbitrary precision. Sampled values from such ascreen space velocity map could be mapped to a pixel value at a givendisplay resolution as needed. A screen space velocity map (also referredto interchangeably as a pixel velocity map when referring to pixels inan animation frame) may show a measured velocity (i.e., a rate anddirection of movement) for points in the screen space over a specifiedtime step (e.g., from one frame to the next). However, the velocity isdetermined relative to changes in the 3D model from one frame to thenext. More specifically, the software tool may determine how much aparticular pixel (or feature) corresponding to a point in a 3D modelmoves over an arbitrary time sample. Thus, the velocity of a pixel (orother sampled point in screen space) represents the direction andmagnitude of movement of that pixel based on changes in the underlying3D geometry from frame to frame (or other time step).

When an artist composites (or blends) two-dimensional (2D) image data onan image rendered from the 3D model, e.g., by drawing some line work,the velocity of a pixel in screen space (based on changes in theunderlying 3D model over time) can be used to effectively animate the 2Dimage data in subsequent (or previous) frames (or over other timesteps). Further, the pixel velocity data can be rendered as color mapsused to provide an artist with a measure of how a much the pixels in ananimation frame are changing from one frame to another.

In another embodiment, the software tool may be configured to generate a2D silhouette (referred to as a silhouette feature, curve, or justsilhouette) corresponding to the outline of a character or object in arendered animation frame. Such a 2D silhouette tracks not with thesurface of the character (as in texture-mapping), but rather with atangent edge of that object as seen from the camera position in anygiven point in time. The derived silhouette remains stable over timewith respect to the geometry of the 3D model, even as the silhouettefeature slides around the surface of the 3D model, due to the changingpositions of the model surface and the camera.

In one embodiment, to generate the silhouette feature, a continuous 2Dsurface parameterization is defined for a portion of geometry in a 3Dgraphical model. For example, the continuous 2D surface parameterizationcan be represented topologically as a sculpted cylinder which contoursitself to the surface of the 3D geometry. The resulting sculptedcylinder may be simplified to define a series of rings. Each ringprovides a closed loop around the surface of the 3D model. The height ofeach ring (relative to the sculpted cylinder) provides a first dimensionof the 2D surface parameterization. Up to two points on each ring eachprovide a second dimension of the 2D surface parameterization.Specifically, one (or two) points on each ring are identified which aretangent to a viewpoint of the camera, one on either side of the surfaceof the 3D geometry enveloped by given ring. These points represent the“edge” of either side of the 3D object for an animation frame identifiedbased on the rendering of the 3D geometry and the position of thecamera. Connecting each of the tangent points derived from the pluralityof rings provides the 2D silhouette curve. The silhouette feature may betracked over one or more frames. As the 3D model is animated, thesilhouette feature may slide across the surface of the model, butnevertheless remain coherent as tracing the edge of the object.

Further, the resulting 2D silhouette curves may be extrudedperpendicular to both the surface of the model and the camera'sline-of-sight to make a ribbon, or flange. Using this method, the flangeslides around the 3D model in reaction to the object or camera'schanging motion, but does not slide up or down the sculpted cylinder.That is, while the tangent points may move around the closed loop of thering on the surface of the object, the ring itself remains generallyfixed in place on the character object. Thus, the ribbon provides astable platform for any kind of silhouette-based artistic effect. Forexample, the ribbons of the silhouette may be rendered using a grainycharcoal effect to give the appearance of hand drawn line work, but onewhere the graininess remains much more constant from frame-to-frame.Similarly, the flange provides a small canvas which can guide a user inaugmenting a rendered image with hand drawn line work or other 2D imagedata. Further, in either case, the velocity of points on the silhouetteribbon (i.e., a magnitude and direction of movement of over an arbitrarytime step, such as from frame to frame) can be generated for the flange.Doing so allows the 2D image data of the silhouette (e.g., line work,image data or other silhouette based artistic effect) to be animated ina sequence of frames (moving either forward or backward based onrendering of the 3D geometry of the relevant time steps).

Note, while discussed separately, the techniques for generating pixelvelocity maps, for animating 2D line work and for generating silhouettesmay be combined, to operate in conjunction with one another as well ascombined with other techniques for generating (or modifying) animationframes. For example, as just noted, the approaches for determining pixelvelocity and animating 2D image data may be applied to the 2D silhouetteribbons generated for an animated 3D character. Further, the examplesused below to illustrate embodiments of the invention generally use therendered appearance of an animated “character.” However, one of ordinaryskill in the art will recognize that the techniques described herein arein no way limited to use with anthropomorphic characters, and insteadmay be used to create combinations of CGI renderings and hand drawn linework, and silhouettes, for any underlying 3D geometry.

Additionally, the following description references embodiments of theinvention. However, it should be understood that the invention is notlimited to specific described embodiments. Instead, any combination ofthe following features and elements, whether related to differentembodiments or not, is contemplated to implement and practice theinvention. Furthermore, although embodiments of the invention mayachieve advantages over other possible solutions and/or over the priorart, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the invention. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s). Likewise, reference to“the invention” shall not be construed as a generalization of anyinventive subject matter disclosed herein and shall not be considered tobe an element or limitation of the appended claims except whereexplicitly recited in a claim(s).

Aspects of the present invention may be embodied as a system, method orcomputer program product. Accordingly, aspects of the present inventionmay take the form of an entirely hardware embodiment, an entirelysoftware embodiment (including firmware, resident software, micro-code,etc.) or an embodiment combining software and hardware aspects that mayall generally be referred to herein as a “circuit,” “module” or“system.” Furthermore, aspects of the present invention may take theform of a computer program product embodied in one or more computerreadable medium(s) having computer readable program code embodiedthereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus or device.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality and operation of possible implementations ofsystems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. Each block of the block diagrams and/orflowchart illustrations, and combinations of blocks in the blockdiagrams and/or flowchart illustrations can be implemented byspecial-purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

FIG. 1 illustrates a computing infrastructure 100 configured to providean artist with an animation tool 110, according to one embodiment of theinvention. As shown, the computing infrastructure 100 includes arendering system 105, a client system 130, and a source of 3D modelgeometry 115, each connected to a communications network 120.Illustratively the rendering system 105 includes an animated video 125generated from the 3D model geometry 115. The rendering system 105generally represents a computing system configured to generate animationframes (i.e., a 2D array of pixel color values) from 3D model geometry115, typically under the direction and control of a user interactingwith the animation tool 110. While FIG. 1 includes a single renderingsystem 105, sophisticated rendering environments typically include manysuch systems rendering different frames of animation simultaneously.Further, while shown as distinct computing systems, the functions of theanimation tool 110, rendering system 105 and storage for the 3D modelgeometry 115 may be combined on a single computing system (or furthersubdivided across multiple computing systems).

In this particular example, client system 130 represents a computersystem executing the animation tool 110. The animation tool 110 may beused create and edit the 3D model geometry 115, as well as to controlthe rendering of such geometry generated by the rendering system 105. Asdescribed in greater detail below, in one embodiment, the animation tool110 may be configured to generate pixel velocity maps as well ascomposite line work drawn by an artist on frames of the animated video.The animation tool 110 may also be configured to transform such linework and composite it with subsequent (or previous) frames based on thepixel velocity maps generated by the animation tool 110. Further, theanimation tool 110 may be configured to generate a silhouette or ribbonlike flange for objects in the animated video 125.

FIG. 2A illustrates an example of a character 205 rendered from athree-dimensional (3D) model, according to one embodiment of theinvention. In this particular example, the rendered frame shows an imageof a female character 205. Assume for this example, the 3D geometrycorresponding to character 205 provides a 3D graphical model of thischaracter running down a city street generally from right to left acrossa display screen when fully rendered. When running, portions ofcharacter 205 change position at different rates from frame to frame.For example, as shown in FIG. 2A the front leg will move faster than thetorso of character 205, and thus have a greater pixel velocity inrendered screen space, until making a footfall, at which point the otherleg will begin to exhibit a higher right moving velocity.

More generally, elements of 3D geometry used to render a frame (e.g., avertex of a polygonal mesh representing the character 205) will changepositions from one frame to the next (or previous) or over other timesteps. In response, the rendered frames of this 3D model will changeaccordingly as well. In one embodiment, an animation tool may beconfigured to generate a pixel velocity map representing the dynamicvelocity of each pixel in a rendered frame, relative to changes in the3D model determined over an arbitrary time step (e.g., from one frame tothe next). That is, a pixel velocity map represents how much one pixelmay be moving (including both a magnitude and a direction) within screenspace from one frame to the next (or to the previous) frame, where themagnitude and direction of the pixel velocity in screen space is derivedrelative to changes in the underlying 3D model.

For example, FIGS. 2B-2C illustrate graphical representations of ascreen space velocity maps corresponding to the character rendered inthe animation frame shown in FIG. 2A, according to one embodiment of theinvention. In one embodiment, a screen space, or pixel velocity map mayrepresent velocity for points in screen space (e.g., pixels) usinggrayscale values, e.g., where brighter pixels represent greatervelocity. Alternatively, by adding a color dimension, both the directionand magnitude of velocity of points in screen space may be representedin a velocity map. For example, FIG. 2B shows, a screen space velocitymap 220 where a rendered color of a pixel presents a measure of pixelvelocity for that pixel in that frame. Note, the velocity may bemeasured for an arbitrary forward or backward time step. In thisexample, the velocity is presumed to measure pixel velocity for acurrent frame relative to a previous frame. Also in this example, thehorizontal velocity of a pixel is represented using the color red, andthe brightness of a given pixel corresponds to the magnitude ofhorizontal movement. That is, greater rightward velocity results in abrighter red color for a pixel in the pixel map (up to the displaylimits of a display screen). At the same time, the vertical velocity ofa pixel is represented using the color green, where pixels moving up ina vertical direction are colored green, again where the brightness of agiven pixel corresponds to the magnitude of vertical movement. Therelative contribution the movement of a pixel in the rightward andupward direction leads to a composite where a pixel moving up and rightis colored yellow, with a brightness determined by the combinedmagnitudes of moment. Note, pixel velocities to the left or down do notcontribute to the composite color (or brightness) of a pixel in thisexample. Of course, other colors could be chosen, the directions couldbe changed and colors could be assigned for horizontal movement to theleft and vertical movement down. By rendering pixels using a colorselected as a composite of vertical and horizontal velocity, pixelvelocity map 220 shows the general movement and direction of each pixelof the character 205 for pixel velocities moving up and to the right.

Similarly, FIG. 2C illustrates a screen space velocity map 240 that usescolor to represent the direction of pixel velocity. However, in additionto a color representing direction, the magnitude of pixel velocity isalso shown by rendering each pixel as a vector representing an initialposition of that pixel in screen space (based on the underlying geometryof the 3D model) and a subsequent position of that pixel over a timestep, e.g., from one frame to the next. By rendering the change inposition of each pixel from one frame to the next, the pixel velocitymap 240 shows both the magnitude and direction of changes to the pixelsin screen space, as the underlying 3D model is used to render frames.For example, a pixel at point 242 is shown. The movement of this pixel(i.e., its velocity) in a particular direction over a time step is shownat an end position 244. The effect results in a pixel velocity map 240where the general appearance of the pixel velocity vectors stack toresemble a collection of dowels. Each such dowel represents a pixelmoving with a given magnitude and direction. As with FIG. 2B, the colorand brightness of each pixel velocity vector is determined relative tomovement of a pixel up and to the right.

FIG. 3 illustrates a method 300 for generating screen space velocitymaps, such as the maps shown in FIGS. 2B-2C, according to one embodimentof the invention. As shown, the method 300 begins at step 305 where the3D geometry of a model is projected into screen space, relative toelements of the model visible to a rendering camera. For example, arendering application may receive a 3D model defined using a polygonalmesh and generates animation frames based on a defined position of acamera and the topology of the 3D model. Each rendered image of thegeometry generally corresponds to an animation frame.

At step 310, the animation tool may calculate velocities for coordinatelocations (e.g., pixels) in screen space in positive (and/or negative)time steps. As noted, the screen space velocities are determinedrelative to changes in the 3D model over the specified time steps (e.g.,from frame-to-frame) but translated to provide a pixel velocity relativeto changes in screen space. For example, the direction and magnitude ofmovement of a given pixel is measured in screen pixels, as determinedrelative to changes in the underlying 3D model over the specified timesteps. If the screen space is sampled using a resolution that differsfrom a display screen, then the resulting screen space velocity map maybe normalized to the resolution of a display screen. For example, if ascreen space is sampled over a continuous range from 0 to 1 (to somedecimal point), then the point velocity values can be mapped into apixel space resolution. At step 315, the animation tool may store thecalculated velocity for the pixels over each specified time step. Thatis, the animation tool may store the screen space velocity mapsgenerated for each time step.

In one embodiment, e.g., a user may specify an animation sequenceencompassing a specified number of frames. From a specified startingframe, the animation tool renders (or causes to be rendered) a number ofsubsequent and/or previous frames. A pixel velocity map may be generatedto identify pixel velocities between any two frames in the sequence.Once generated, the animation tool may store the resulting pixelvelocity maps. At step 320, the animation tool may create arepresentation of a pixel velocity map. For example, pixel velocity maybe represented using colors (as shown in FIG. 2B) or as a set of vectorsshowing the direction and magnitude of pixel velocity (as shown in FIG.2C). Of course, pixel velocity may be represented using a variety ofdifferent colors schemes, grayscale values, intensity values, or codedpatterns, etc.

Further, in one embodiment, the pixel velocity maps may be used toanimate 2D image data, line work or other 2D artistic effects compositedwith an animation frame by an artist over the specified time steps. Forexample, FIGS. 4A-4B illustrate example animation frames rendered from athree-dimensional (3D) model which include hand drawn 2D line work drawnon one frame rendered in subsequent frames using pixel velocity maps,according to one embodiment of the invention. As shown, a renderedcharacter 400 is running down a city street, where the legs of thecharacter 400 show a full stride in progress. As a result, much of thesurface of the skirt worn by character 400 is visible in this frame.Further, as shown, an artist has added hand drawn line work to therendered clothing of the character 400. Specifically, a line 405 and acircle 410 have been drawn on the dress of the character in theanimation frame shown in FIG. 4A.

The animation tool may generally be configured to composite the linework supplied by the artist with the animation frame. Conventionally, anartist would have to add line work to each subsequent frame to continuethe effect. In one embodiment, however, the pixel velocity maps are usedto animate the 2D line work (e.g., line 405 and circle 410) insubsequent (or previous) frames. For example, FIG. 4B represents a framerendered several time steps after the frame shown in FIG. 4A. As shownin FIG. 4B, the character 400′ has reached a curb and both feet arepositioned relatively close together. As a result, the visible surfacearea of the skirt on character 400′ is much smaller. Further, the pixelvelocities corresponding to the pixels rendered from the 3D modelcovered by the line 405 and the circle 410 have been used to determine amagnitude and direction to move each pixel of the line 405′ and thecircle 410′ over each time step, resulting in a bent line and deformedcircle.

More specifically, for each time step of the rendered animation (e.g.,for each frame between the frame shown in FIG. 4A and the frame shown inFIG. 4B), the pixel velocity map is used to translate elements of 2Dimage data, e.g., the line work of the line 405 and circle 410 from aninitial position to an ending position. Accordingly, the shape of theline 405′ and the circle 410′ have changed in a manner generallyconsistent with the changes in the shape of the skirt rendered oncharacter 400. Thus, the pixel velocity maps are used to effectivelyanimate the 2D image data supplied by the artist over the subsequent (orprevious) time steps, without requiring input from the artist. Ofcourse, the artist could also manually edit the resulting 2D image datagenerated by the pixel velocity maps if needed to create a desiredappearance or aesthetic quality.

FIG. 5 illustrates a method 500 for rendering hand drawn 2D line workusing pixel velocity maps, according to one embodiment of the invention.As shown, the method 500 begins at step 505 where the animation toolrenders (or causes to be rendered) a sequence of animation frames. Atstep 510, the rendered frames are represented by a user interacting withan editing tool. Such a tool may be configured to allow the user to stepthrough the rendered frames one by one. At step 515, the user maycomposite 2D image data with one of the frames using a drawing tool. Atstep 520, the animation tool may generate (or retrieve) screen spacevelocity maps for each time step (e.g., a pixel velocity map generatedto represent pixel velocity between each animation frame). At step 525,the animation tool may animate the 2D image data provided for one framein one or more successive (or preceding) frames using the screen spacevelocity maps. That is, each element of 2D line work or other 2D imagedata is translated by a direction and magnitude determined using thescreen space velocity maps. Doing so animates the 2D image data in amanner consistent with changes in the underlying 3D geometry. Theresulting 2D image is composited with the animation frames generated foreach successive (or preceding) time step.

As noted above, in addition to generating screen space or pixel velocitymaps, the animation tool may be configured to generate a 2D silhouettefor a character or other object depicted in a sequence of animationframes. Unlike texture mapping, where a texture is applied to thesurface of an object, the silhouette aligns with a tangent edge of thatobject as seen from the camera position at any given point in time. The2D silhouette remains stable over time with respect to the geometry ofthe 3D model, even as the shape of the 2D silhouette “slides” around thesurface of the 3D model, due to the changing positions of the modelsurface and the camera. Further, a variety of 2D graphic effects may beapplied to the 2D silhouette as needed to create a silhouette for anobject having a desired aesthetic appearance. Further still, bygenerating a screen space velocity map for a rendered image, the 2Dsilhouette, line work, or other 2D image data applied to the silhouettemay be animated in the same manner as described above for image datasupplied by an artist.

FIG. 6 illustrates a method for generating a 2D silhouette feature forgeometry of a 3D graphical model rendered in a sequence of animationframes, according to one embodiment of the invention. Aspects of themethod 600 are described in conjunction with FIGS. 7A-7B, whichillustrate an example of generating a silhouette flange for a 3Dgraphical model. As shown, the method 600 begins at step 605, where auser interacts with the animation tool to define a cylindrical featurecorresponding to elements of 3D geometry in a 3D graphical model. Thecylindrical feature is used to derive a 2D silhouette defining thevisible edge of 3D graphical model for each of a sequence of animationframes. For example, the animation tool may include a drawing tool usedto “paint” a cylinder onto a portion of 3D model geometry. In response,the animation tool may “fit” the cylinder to the 3D geometry of theobject in each animation frame rendered from the geometry. For example,the animation tool may shrink the cylinder until it intersects with thesurface of the 3D geometry in each frame.

Once the topology of the resulting cylindrical feature wrapping the 3Dgeometry is determined, a loop begins that includes steps 615-630. Witheach pass through the loop, the animation tool generates geometrycorresponding to a 2D silhouette tracing the visible edge of thegeometry in a given frame. More generally, the cylindrical feature isused to derive continuous 2D surface parameterization for a portion ofgeometry in a 3D graphical model defining a silhouette in each frame. Atstep 610, the animation tool may subdivide the cylinder into a series ofrings. The number of rings and/or the distance between rings may be setas a matter of preference. Each ring provides a closed loop wrappingaround the surface of the 3D model, generally perpendicular to the axisthe cylindrical feature. The position of each ring provides a firstparameter for the 2D parameterization of the silhouette feature.

For example, FIG. 7A shows a character 705—in this case a femalecharacter standing upright. An artist has used the animation tool tospecify the boundary for a cylinder as generally corresponding to theleft leg (from the perspective of the artist) of character 705. Inresponse, the animation tool has identified a cylindrical feature thatintersects the surface of this leg and subdivided this feature intorings over relatively short intervals along the axis of the cylinder.For example, beginning at ring 710, FIG. 7A shows a series of ringsmoving upwards from ring 710 until the geometry of the leg intersectsthe torso of character 705. To create a complete silhouette, additionalcylinders could be defined for the other leg, each arm, torso and headof the character 705. As noted the position (e.g., the height along thecylinder) of each ring on the 3D geometry provides a first parameter forthe 2D parameterization of a silhouette curve.

Referring again to method 600, at step 615 two points on each ring areused to identify a point on a left and right (or top and bottom) curverepresenting a 2D silhouette of the character for each frame ofanimation generated from the 3D model. Specifically, in each frame, eachring has exactly two points tangent to the viewpoint of a renderingcamera—one of the left and right (or top and bottom) of each ring. Forexample, FIG. 7A shows the rings 715 and tangent points determined forleft and right sides of the leg of the character 705. As shown, eachring includes a left and right point, again where each point isdetermined as a point on the ring tangent to the viewpoint of thecamera. The tangent points provide the second parameter for the 2Dparameterization of a silhouette curve.

FIG. 8 shows an additional example. More specifically, FIG. 8illustrates an example of tangent points being identified for points ona 3D model, according to one embodiment of the invention. As shown in atop down view, a rendering camera 805 shows the head and shoulders ofcharacter 705. The character is within a viewing plane 810 of thecamera. In this example, assume a cylinder has been defined for the headand torso of character 705. Tangent lines 815 ₁₋₂ show tangent pointsfor the torso cylinder and tangent lines 820 ₁₋₂ show tangent points forthe head.

Referring again to method 600, at step 620, the animation tool connectsthe tangent points to create left and right (or top and bottom)silhouette curves. Each such curve provides a 2D curve that traces theedge of the 3D geometry, as that geometry is rendered in a given frameof animation. Additionally, at step 625, the animation tool may extrudeeach curve to create ribbons, or a flange, of 2D geometry providing asilhouette of the character.

In one embodiment, the tangent points corresponding to each of the twocurves are extruded perpendicular to the surface of the model and thecamera's line-of-sight to make a ribbon, or flange. This result is shownin FIG. 7B where a ribbon 720 ₁₋₂ connects the tangent points extendedalong the surface normals of each such point for both the left and rightside of the leg of the character 720. A complete set of tangent ribbonsfor character 705 is shown, without the character geometry, at 725. Theribbons provide a collection of 2D image data that may be compositedwith the rendered animation frames. Note, given the topology of the 3Dgeometry in a given frame, some portions of the silhouette curve (orribbon flange extruded therefrom) may be occluded by other elements of3D geometry. For example, as shown at 730, the ribbon silhouettecorresponding to the right arm of the character 705 occludes a portionof the silhouette corresponding to the right hip of character 705.

Another example of a generating a silhouette curve is shown in FIGS.9A-9B. More specifically, FIGS. 9A-9B illustrate an example of derivingtangent points along a cylinder to generate a silhouette flange for a 3Dmodel of a dancing female character, as rendered in two differentanimation frames. As shown, character 905 is generally facing the cameraand much of the leg with the cylinder rings is occluded by the otherleg. In contrast, character 905 has turned, clearly revealing the legwith the cylinder ring. As these images demonstrate, the rings of agiven cylinder stay temporally coherent, despite dramatic changes in thegeometry of the rendered object.

At the same time, points on the cylinder rings tangent to the camerachange from frame to frame. That is, the shape of the silhouette itselfslides across the character as the tangent points of the rings changefrom frame to frame. This result in shown in FIGS. 10A-10B, whichillustrates the silhouette flange shown for the dancing female characterfrom FIGS. 9A-9B using an alternative camera angle. More specifically,FIGS. 10A-10B show a view of the dancing character with a camera fixedin the front of the character for both frames. However, the silhouetteflanges 1020 ₁₋₂ correspond to a silhouette generated for the cameraangles used in FIGS. 9A-9B. As shown by these figures, while thesilhouette itself remains temporally coherent along the edges of thecharacter across different frames (as determined using the tangentpoints on the rings), the position of the silhouette edges or ribbonwill slide along the surface of the 3D geometry as the object changes inshape or position within 3D space (or as the camera moves) from frame toframe (or over some other time step).

Referring again to method 600, at step 630 the animation tool maygenerate 2D image data to composite with the rendered animationframes—effectively adding a visible silhouette to animation framesrendered from 3D geometry. Further, the 2D image data may be generatedto have a variety of different visual characteristics, e.g., differentcolors, thickness or line styles, as well as though it had been drawnusing different media, ink, pencil, charcoal, paint, etc. For example,FIGS. 11A-11B illustrate a 2D silhouette of the 3D model shown in FIGS.9A-9B, according to one embodiment of the invention. In this example,the silhouette of the dancing character 1105 and 1110 is drawn using aline style that mimics a hand drawn appearance, where potions of theline work vary in thickness and darkness.

Similarly, the silhouette ribbons provide a 2D drawing canvas for anartist to supply line work or other 2D image data which can becomposited with the image frames generated from the 3D image data. Such2D image data supplied for the silhouette of one frame can be animatedin subsequent (or previous frames) using the pixel velocity mapsdescribed above. The resulting 2D animation data can be composited withsubsequent animation frames to create a desired visual effect

FIG. 12 illustrates an example computing system 1200 configured toimplement embodiments of the invention. As shown, the computing system1200 includes, without limitation, a central processing unit (CPU) 1205,a network interface 1215, a bus 1217, a memory 1220, and storage 1230.The computing system 1200 also includes an I/O device interface 1210,connecting the computing system 1200 to I/O devices 1212 (e.g.,keyboard, display and mouse devices). The computing system 1205 providesa computing system which allows a user to generating pixel velocitymaps, for animating 2D line work and for generating 2D silhouettes forframes of rendered 3D geometry.

CPU 1205 retrieves and executes programming instructions stored in thememory 1220. Similarly, CPU 1205 stores and retrieves application dataresiding in the memory 1220. The bus 1217 is used to transmitprogramming instructions and application data between the CPU 1205, I/Odevices interface 1210, storage 1230, network interface 1215, and memory1220. CPU 1205 is included to be representative of a single CPU,multiple CPUs, a single CPU having multiple processing cores, and thelike. And the memory 1220 is generally included to be representative ofa random access memory. The storage 1230 may be a disk drive storagedevice. Although shown as a single unit, the storage 1230 may be acombination of fixed and/or removable storage devices, such as magneticdisc drives, solid state drives (SSD), removable memory cards, opticalstorage, network attached storage (NAS), or a storage area-network(SAN).

Illustratively, the memory 1220 includes the animation tool 1221, whichitself includes a 3D modeling component 1222, a pixel velocity mapcomponent 12223, a 2D line work editing component 1224 and a silhouettegenerating component 1225. And storage 1230 includes 3D model data 1231,rendered animation frames 1232, pixel velocity maps 1233, 2D line work1234 and silhouette data 1235. Of course, one of ordinary skill in theart will recognize that the functional components of the animation tool1221 can be implemented using a variety of approaches.

As described above, the 3D modeling component 1222 allows a user torender 3D model data 1231, e.g., to generate a sequence of renderedanimation frames 1232. Further, once a set of frames have been rendered,the pixel velocity map 1223 may be used to derive pixel velocity maps1233 for the rendered animation frames 1232. Further still the 2D linework component 1224 may allow an artist to draw 2D line work 1234composed with rendered animation frames. The 2D line work component 1224may also be configured to animate the line work over one or more timesteps in a forward or backward direction. The silhouette component 1225may be configured to generate silhouette data 1235 for the renderedanimation frames 1232.

Advantageously, embodiments of the invention provide techniques forcreating animated video frames which include both computer generatedelements and hand drawn elements. For example, one embodiment includes asoftware tool which allows an artist to draw line work (or supply other2D image data) to composite with an animation frame rendered from athree dimensional (3D) graphical model of an object. The software toolmay be configured to determine how to animate such 2D image dataprovided for one frame in order to appear in subsequent (or prior)frames in a manner consistent with changes in rendering the underlying3D geometry.

The software tool may also be configured to generate a silhouette curvecorresponding to the outlines of a character or object in a renderedanimation frame. That is, the software tool may be configured to derivea 2D silhouette which traces the edges of an object in animation framesrendered from a 3D model. By extruding points along the 2D silhouettecurve, a variety of graphical effects may be applied to the silhouette.Further, any 2D image data or line work may be animated using pixelvelocity maps generated from the underlying 3D model.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A computer-implemented method for processing an animation frame, themethod comprising: identifying a first animation frame rendered by oneor more processors, wherein the first animation frame rendersthree-dimensional (3D) geometry corresponding to a 3D model object at afirst point in time; and generating a screen space velocity mapindicating a dynamic velocity of one or more coordinates in the firstanimation frame relative to changes in points on the 3D model objectoccurring over one or more specified time steps.
 2. The method of claim1, wherein the one or more coordinates in the screen space velocity mapare mapped to pixel coordinates in the first animation frame, andwherein the dynamic velocity of each coordinate includes both amagnitude and direction of pixel movement.
 3. The method of claim 2,further comprising: generating a visual representation of the screenspace velocity map, wherein the visual representation encodes amagnitude and direction of pixel movement using a specified colorscheme.
 4. The method of claim 1, further comprising: receivingtwo-dimensional (2D) image data to composite with the first animationframe; and animating the 2D image data over the specified time stepaccording to the dynamic velocity of the one or more coordinates in thescreen space velocity map.
 5. The method of claim 1, wherein the one ormore time steps move forward through a rendered animation of the 3Dgeometry.
 6. The method of claim 1, wherein the one or more time stepsmove backward through a rendered animation of the 3D geometry.
 7. Acomputer-readable storage medium storing a program, which, when executedby a processor performs an operation for processing an animation frame,the operation comprising: identifying a first animation frame renderedby one or more processors, wherein the first animation frame rendersthree-dimensional (3D) geometry corresponding to a 3D model object at afirst point in time; and generating a screen space velocity mapindicating a dynamic velocity of one or more coordinates in the firstanimation frame relative to changes in points on the 3D model objectoccurring over one or more specified time steps.
 8. Thecomputer-readable storage medium of claim 7, wherein the one or morecoordinates in the screen space velocity map are mapped to pixelcoordinates in the first animation frame, and wherein the dynamicvelocity of each coordinate includes both a magnitude and direction ofpixel movement.
 9. The computer-readable storage medium of claim 8,wherein the operation further comprises: generating a visualrepresentation of the screen space velocity map, wherein the visualrepresentation encodes a magnitude and direction of pixel movement usinga specified color scheme.
 10. The computer-readable storage medium ofclaim 7, wherein the operation further comprises: receivingtwo-dimensional (2D) image data to composite with the first animationframe; and animating the 2D image data over the specified time stepaccording to the dynamic velocity of the one or more coordinates in thescreen space velocity map.
 11. The computer-readable storage medium ofclaim 7, wherein the one or more time steps move forward through arendered animation of the 3D geometry.
 12. The computer-readable storagemedium of claim 7, wherein the one or more time steps move backwardthrough a rendered animation of the 3D geometry.
 13. A system,comprising: a processor; and a memory, wherein the memory includes anapplication program configured to perform an operation for evaluatingpixels in an animation frame, the operation comprising: identifying afirst animation frame rendered by one or more processors, wherein thefirst animation frame renders three-dimensional (3D) geometrycorresponding to a 3D model object at a first point in time, andgenerating a screen space velocity map indicating a dynamic velocity ofone or more coordinates in the first animation frame relative to changesin points on the 3D model object occurring over one or more specifiedtime steps.
 14. The system of claim 13, wherein the one or morecoordinates in the screen space velocity map are mapped to pixelcoordinates in the first animation frame, and wherein the dynamicvelocity of each coordinate includes both a magnitude and direction ofpixel movement.
 15. The system of claim 14, wherein the operationfurther comprises: generating a visual representation of the screenspace velocity map, wherein the visual representation encodes amagnitude and direction of pixel movement using a specified colorscheme.
 16. The system of claim 13, wherein the operation furthercomprises: receiving two-dimensional (2D) image data to composite withthe first animation frame; and animating the 2D image data over thespecified time step according to the dynamic velocity of the one or morecoordinates in the screen space velocity map.
 17. The system of claim13, wherein the one or more time steps move forward through a renderedanimation of the 3D geometry.
 18. The system of claim 13, wherein theone or more time steps move backward through a rendered animation of the3D geometry.
 19. A computer-implemented method for generating atwo-dimensional (2D) silhouette for geometry of a 3D graphical model,the method comprising: defining a continuous surface parameterizationfor at least a portion of the geometry representing the 3D graphicalmodel, wherein a first dimension of the 2D surface parameterizationintersects a portion of a surface of the 3D graphical model; tracking asilhouette feature over one or more frames, wherein the trackingincludes: identifying a plurality of points on the first dimension ofthe surface parameterization, for each of identified point, determiningat least one second dimension of the surface parameterization, eachproviding a tangent point on the surface of the 3D model tangent to aviewpoint of a rendering camera, and connecting the tangent points toproduce geometry defining a 2D silhouette for the portion of the surfaceof the 3D graphical model in each of the one or more frames.
 20. Themethod of claim 19, wherein the first dimension of the 2D surfaceparameterization is determined as a cylindrical feature wrapping aportion of the surface of the 3D graphical model.
 21. The method ofclaim 20, further comprising, deriving a plurality of closed loopsgenerally perpendicular to an axis of the cylindrical feature, andwherein determining at least one second dimension of the surfaceparameterization comprises identifying at least one point on the closedloop tangent to the viewpoint of the rendering camera.
 22. The method ofclaim 19, further comprising, extruding the tangent points in adirection perpendicular to both the surface of the 3D graphical modeland a line-of-sight of the rendering camera to generate a 2D ribbonsilhouette tracing an edge of the 3D graphical model in each of the oneor more frames.
 23. The method of claim 22, further comprising: applyinga graphical effect to the 2D ribbon silhouette tracing the edge of the3D graphical model; generating screen space velocity maps indicating adynamic velocity of one or more coordinates in the 2D ribbon silhouetterelative to changes in points on the 3D model object occurring over eachof the one or more frames; and animating the 2D ribbon silhouette overeach of the one or more frames according to the dynamic velocity of theone or more coordinates in the screen space velocity maps.
 24. Themethod of claim 22, further comprising, compositing the animated 2Dribbon silhouette with the one or more frames.
 25. A computer-readablestorage medium storing a program, which, when executed by a processorperforms an operation for generating a two-dimensional (2D) silhouettefor geometry of a 3D graphical model, the operation comprising: defininga continuous surface parameterization for at least a portion of thegeometry representing the 3D graphical model, wherein a first dimensionof the 2D surface parameterization intersects a portion of a surface ofthe 3D graphical model; tracking a silhouette feature over one or moreframes, wherein the tracking includes: identifying a plurality of pointson the first dimension of the surface parameterization, for each ofidentified point, determining at least one second dimension of thesurface parameterization, each providing a tangent point on the surfaceof the 3D model tangent to a viewpoint of a rendering camera, andconnecting the tangent points to produce geometry defining a 2Dsilhouette for the portion of the surface of the 3D graphical model ineach of the one or more frames.
 26. The computer-readable storage mediumof claim 25, wherein the first dimension of the 2D surfaceparameterization is determined as a cylindrical feature wrapping aportion of the surface of the 3D graphical model.
 27. Thecomputer-readable storage medium of claim 26, wherein the operationfurther comprises, deriving a plurality of closed loops generallyperpendicular to an axis of the cylindrical feature, and whereindetermining at least one second dimension of the surfaceparameterization comprises identifying at least one point on the closedloop tangent to the viewpoint of the rendering camera.
 28. Thecomputer-readable storage medium of claim 25, wherein the operationfurther comprises, extruding the tangent points in a directionperpendicular to both the surface of the 3D graphical model and aline-of-sight of the rendering camera to generate a 2D ribbon silhouettetracing an edge of the 3D graphical model in each of the one or moreframes.
 29. The computer-readable storage medium of claim 28, whereinthe operation further comprises: applying a graphical effect to the 2Dribbon silhouette tracing the edge of the 3D graphical model; generatingscreen space velocity maps indicating a dynamic velocity of one or morecoordinates in the 2D ribbon silhouette relative to changes in points onthe 3D model object occurring over each of the one or more frames; andanimating the 2D ribbon silhouette over each of the one or more framesaccording to the dynamic velocity of the one or more coordinates in thescreen space velocity maps.
 30. The computer-readable storage medium ofclaim 28, wherein the operation further comprises, compositing theanimated 2D ribbon silhouette with the one or more frames.